Dry electrical recording medium



United States Patent 2,776,252 DRY ELECTRICAL RECORDING MEDIUM Robert S.SchwartnGreat Neck, N. Y., assig uor to Faximile, lnc New York, N. Y., acorporation of Delaware No Drawing. Application April 23, 1953,

Serial No. 350,757

4 Claims. or. 204-4 This invention relates to recording media of thetype which is subjected to an external application of heat to render themedium markable electrically.

While not limited thereto, the invention is particularly useful in theart of electrical recording wherein it is desirable to pass a recordingcurrent through a conductive recording medium containing anelectro-responsive marking compound. In recorders operating on thisprincp le, the recording current is applied to two cooperatingelectrodes, and a conductive sheet or tape, which may be impregnatedpaper or cloth, is drawn between the electrodes. Marks are made on theconductive medium by electrochemical change.

Paper, being white and inexpensive, is very desirable for recordpurposes. However, paper is normally nonconductive and must beimpregnated with suitable chemicalsto make it electro-conductive andsuitable for use in electrical recorders. Perhaps the most successful ofthe recording papers in use at the present time is the type impregnatedwith an aqueous electrolyte and a marking compound such as disclosed inpatent to I. V. L. Hogan et al., 2,339,267. This paper is eminentlysuitable for most recording applications but has the characteristic thatit must be maintained with the proper moisture content during storageuntil actually recorded on by recording apparatus.

Another type of electrographic recording paper is known and disclosed inpatent to R. B. Gibney, 2,530,956. This type of paper is fabricated as acomposite sheet of several layers. One layer, the recording layer, isimpregnated with a solid compound consisting of acetamide alone or incombination with propionamide or formamide. Another layer is impregnatedwith the same amide or amdies and an ionizable organic dye, the ions ofwhich migrate to the recording layer when the solid impregnant is meltedby the passage of an electric current therethrough to produce acharacteristic mark. In recording paper of this type there is thedisadvantage that at least two layers are required to form the compositesheet. The acetamide which forms the basic impregnant sublimes orevaporates readily at normal ambient temperatures and pressures so thatthe paper must be stored in a sealed container in an acetamide vapor toremain operative. At ambient relative humidities exceeding 85% the paperbecomes soft, weak, sticky and wrinkled because the ,amide content isextremely hygroscopic. The high humidity renders the paper unmanageablein conventional recorders and markability becomes either impaired orimpossible.

Applicants purpose has been to provide a dry, white recording mediumemploying a single layer sheet or tape of paper or cloth impregnatedwith one or more suitable amides, an ionizable salt, and anelectro-responsive substantially colorless marking compound. Therecording medium is markable only by being first externally heated andthen subjected to a suitable electrical voltage between electrodes, oneor which may be an element having an iron content. The recording mediumis characterized by its extreme stability under conditions of highhumidity and prolonged exposure to the ambient atmosphere.

, It is therefore a principal object of-the invention to 2 provide a dryrecording medium which can be rendered conductive only by externalheating and is then electrically markable.

It is a further object to provide a White dry, single layer porousrecording medium impregnated with a primary or secondary amide having atleast four carbon atoms, an ionizable salt, and a substantiallycolorless electro-responsive marking compound.

It is a further object to provide a white, dry recording medium which isrendered electro-conductive only when externally heated and whichremains wholly stable and markable under all humidity conditions.

It is a further object to provide a dry recording medium which iselectro-conductive and electrically markable when externally heated andwhich retains its markability on prolonged exposure to ambientatmospheric conditions.

It is a further object to provide a dry recording medium impregnatedwith a solid amide which does not require special packaging for storageprior to use.

It is a further object to provide a novel impregnant composition for arecording medium.

For a description of a recording apparatus and method suitable for therecording medium of this invention reference is had to my copendingapplication Serial No. 102,136, filed June 29, 1949, now Patent No.2,662,803.

Generally the invention contemplates a recording medium such as paper orcloth impregnated with one or more amides having four or more carbonatoms, a highly ionizable salt, and a substantially colorlesselectroresponsive marking compound. The combination of amidle and saltis hereinafter designated as a composite electrolyte. The compositeelectrolyte is normally solid, substantially non-hygroscopic andnon-evaporating under normal ambient conditions. It includes as oneingredient a primary or secondary amide, which may be regarded as anincompletely substituted hydrogen substitution product of ammonia(NI-I3), having a melting point above 20 deg. C. Primary, secondary andtertiary amides may be regarded generally as derivatives of ammonia inwhich one, two, or all three hydrogen atoms in ammonia (NHs) have beenreplaced respectively by one, two, or three (R-CO) or (R-SOz) groups asevidenced by standard works on the subject such as Sidgwicks OrganicChemistry of Nitrogen, edited by Taylor and Baker, Oxford Press, 1937,page 136 et seq. Quite unexpectedly it was discovered that certainprimary and secondary amides having four or more carbon atoms are forthe purpose of this invention considerably superior to other amideshaving less than four carbon atoms. Tertiary amides such asmethyl-acetanilide were found wholly ineffective for purposes of thepresent invention. Since one object of the invention is to provide asubstance which is solid, dry and substantially non-evaporative atordinary ambient temperatures, amides having a low melting point werefound not suitable. Formamide is an example of such an unsuitableprimary amide having less than four carbon atoms since it is a liquid atordinary temperatures and subject to evaporation on exposure. Acetamide,another primary amide having less than four carbon atoms has also provento be unsuitable since it is subject to extreme sublimation orevaporation on exposure and is objectionably hygroscopic in highhumidity ambient conditions. Compositions of acetamide with propionamideand acetamide with formamide have been found to be similarly subject tosublimation or evaporation and are objectionably hygroscopic.

As a general rule normally solid primary and secondary amides containingfour or more carbon atoms have been found to serve as satisfactoryingredients of the composite electrolyte of the present invention sincethey are dry, solid, and substantially non-sublimating or nonevaporatingat normal ambient atmospheric conditions. Among these amides are:acetanalide, para-toluene sulfonamide, butyramide, N-ethyl benzenesulfonamide, and benzamide. The listed amides are not to be taken aslimiting the invention thereto since there is reason to believe otherprimary and secondary amides having four or more carbon atoms would alsoserve the purposes of the present invention.

In addition to an amide of the type described, the normally-solidcomposite electrolyte of this invcr n includes a highly ionizable salt.Numerous highly ionizable salts can be used, such as, for example,sodium nitrate, potassium nitrate, ammonium nitrate, ammonium chloride,ammonium sulfate, and sodium chloride. These salts are not to be takenas limiting the invention which is applicable generally to all highlyionizable salts both inorganic and organic. t will be understood thatwith certain of the salts better results will be obtained with some ofthe amides than with others, and for a particular use some combinationswill be found better than others. Commercial grades of amides maycontain enough saits as impurities to provide electro-conductivity andit will be understood that the use of such impure grades falls withinthe scope of the present invention.

The composite electrolyte may be formed by melting the selected amideand adding a relatively small amount of the salt. Or the amide and thesalt may be dissolved in an appropriate common solvent which may then beremoved by evaporation leaving the dry, substantially white compositeelectrolyte.

In making a dry recording medium which at elevated temperatures ismarkable responsively to electric currents, a suitableelectro-responsive marking compound is added with the salt to the moltenamide. Electro-responsive marking compounds as contemplated l erein aregenerally substantially colorless compounds which are either white orvery light colored. They d pend on electrochemical action to change todark colors or may be heat sensitive so that they decompose to change todark colors. Such compounds are all deemed electro-responsive since theywill darken on passage of an electrical current through them. Among themarking compounds found suitable for purposes of the present inventionare phenols having at least two hydroxy groups in the ortho position,such as catechol, pyrogallol, gallic acid, and tannic acid. Certain heatsensitive electro-responsive marking compounds which have been foundsuitable are lead formate, lead thiosulphate, and nickel formate. Stillother suitable electro-responsive marking compounds are known in theart. Colored dyes are wholly unsuitable for the present invention sincethey cannot be used to impregnate completely the single porous layerwhich carries the impregnant without rendering it useless for markingpurposes.

It may be pointed out that as regards the combination of an amide, asalt and a specific marking compound, the chemical nature of the markingcompound should be taken into consideration and combinations selectedwhich have characteristics appropriate to the requirements of theparticular application involved. it must be emphasized that use of acolored marking dye embodied in a separate layer of a plural layermarking sheet is undesirable in the present invention, since a singlelayer white porous recording medium is required which is capable ofrepeated marking. Multi-layered sheets known in the prior art whereinone layer carries a colored dye and another layer carries anelectrcconduetive impregnant are not capable of repeated markingelectrically. A further advantage of combining a substantially colorlessmarking compound with the composite electrolyte is that manufacture ofthe recording paper is simplified since a single dip in the propersolution impregnates the paper. Multilayered papers requires repeateddips or successive coating steps. The following example is intended asillustrative of the way in which the invention can be carried outemploying primary and secondary amides having four or more carbon atoms:

EXAMPLE 1 About 10 g. of butyramide was melted at about 125 deg. C. with0.1 g. of potassium nitrate and 0.2 g. of catechol. Paper impregnatedwith this melt has a resistance of 2,000 ohms when measured acrossheated electrodes, one inch in diameter pressing the paper therebetweenwith a pressure of 10 pounds per square inch. The dried paper was thenplaced on a hot plate having a temperature of about 120 deg. C.,whereupon it assumed a wet appearance. One terminal of a 45-volt drybattery was connected to the metallic hot plate and the other electrodeto a stylus containing iron. When the stylus was both touched to anddrawn over the paper, intense black marks were left on the paper.

The procedure of Example 1 can also be used to demonstrate themarkability of recording media containing one or more amides having fouror more carbon atoms of which the following are examples:

Para-toluene sulfonamide Benzamide Acetanilide N-ethyl benzenesulfonamide Several additional samples of recording paper were made andtested employing certain other primary, secondary and tertiary amides.It was found that these recording media were ineffective, unstable, orhad other objectionable characteristics as discovered in the testsdescribed below:

EXAMPLE A An inefiective recording medium was made by impregnating paperwith a melt of 10 g. of methyl acetanilide (a tertiary amide which maybe regarded as a completelysubstituted hydrogen substitution product ofammonia), 0.1 g. of potassium nitrate and 0.3 g. of catechol. The paperhad a resistance of 100,000 ohms when measured as in Example 1. Novisible mark was produced when the paper was heated and volts wasapplied across the heated paper through appropriate electrodes as inExample 1. There is reason to believe all other tertiary amides wouldprove similarly ineffective for the purposes of this invention.

EXAMPLE B About 20 g. of acetamide, and 0.3 g. of potassium nitrate wereheated together until molten. Roughly 0.3 g. of catechol was added tothe melt, blotted to remove the excess liquid, and allowed to cool. Theresulting sheet was white and apparently dry. The paper was heated andtested for markability as in Example 1. Black marks were produced on thepaper both when an iron electrode was touched to and was drawn over thepaper.

EXAMPLE C About 20 g. of acetamide, 2 g. of propionamide (a primaryamide having less than four carbon atoms) and 0.3 g. of potassiumnitrate were heated together until molten. Roughly 0.3 g. of catecholwas added to the melt. A piece of absorbent paper stock was immersedinto the melt, blotted to remove the excess liquid and allowed to cool.The resulting sheet was white and apparently dry. The paper was heatedand tested for markability as in Example 1. Black marks were produced onthe paper both when an iron electrode was touched to and was drawn overthe paper.

EXAMPLE D A sample of recording paper was made as described 1n Example Bemploying 20 g. of acctamide, 2 g. of formamide and 0.3 g. of catechol.The paper was heated and tested for markability as in Example 1. Blackmarks were produced on the paper both when an iron electrode was touchedto and was drawn over the paper.

I. Test for hygroscopic efiects Several samples of recording paper werefabricated in accordance with Example 1. They were impregnatedrespectively with butyramide, benzamide, acetanilide, paratoluenesulfonamide, N-ethyl benzene sulfonamide, acetamide, acetamide withpropionamide, and acetamide with formamide. The samples were placed in aclosed humidity test chamber having a relative humidity of at least 85%,at 20 deg. C., and atmospheric pressure. After one hours exposure in thetest chamber the samples were removed. The papers containing amideshaving four or more carbon atoms were all slightly limp from moistureabsorbed by the paper but they were all smooth to touch and unwrinkled.These papers were then heated on a hot plate set at 120 deg. C., andwere tested for markability as in Example 1. Black marks were producedon all the papers containing amides having four or more carbon atomsboth when an iron electrode was touched to and was drawn over thepapers.

The papers containing acetamide were soft, sticky, weak and wrinkledfrom absorbed moisture when removed from the humidity test chamber. Theacetamide papers were heated on a hot plate set at 120 deg. C., andtested for markability as in Example 1. The marks produced when an ironelectrode was touched to and was drawn over the acetamide papers wereirregular, spotty and objectionably light.

ll. Test for efiects of prolonged exposure to the atmosphere Severalsamples of recording paper were fabricated in accordance with Example 1.They were impregnated respectively with the same amides as the paperstested for hygroscopic effects. The samples were then exposed to varyingambient atmospheric conditions of pressure, temperature and humidity fora period of twenty days. Each sample was then heated on a hot plate at120 deg. C., and tested for markability as in Example 1. Black markswere produced on all paper containing amides having four or more carbonatoms both when an iron electrode was touched to and was drawn over thepapers. No mark was produced on the papers containing acetamide. It wasfound that the amide content of the acetamide papers had sublimed orevaporated to the extent that no marking was possible in accordance withthe method of Example 1.

III. Observations and conclusions The above tests clearly demonstratedthat the physical condition of papers containing amides having four ormore carbon atoms fabricated in accordance with Example 1 weresubstantially unaffected by conditions of high humidity and of prolongedexposure to ambient atmospheric conditions, and their markability, whentested in accordance with Example 1 by externally heating and thenapplying an electrical voltage was unimpaired. It was furthermore foundthat the amides in these papers sublimed or evaporated to such a smallextent under all humidity and storage conditions that the markability ofthe papers was not impaired. The papers containing acetamide wereextremely hygroscopic under highly humid conditions, becoming too soft,weak, wrinkled, and sticky to be handled practicably in recordingdevices commonly used in the art. Under conditions of prolonged exposureto ambient atmospheric conditions the amide content of the acetamidepapers sublimed or evaporated to the extent that marking by externalheating and application of an electrical potential in accordance withExample 1 was not possible.

Of the amides found satisfactory as ingredients of the compositeelectrolyte for use in this invention will be noted that all werealiphatic or aromatic primary and secondary amides having four or morecarbon atoms. Of the amides found unsatisfactory all were aliphaticprimary amides, namely: acetamide, acetamide with propionamide, andacetamide with formamides. These are all amides containing less thanfour carbon atoms. Tertiary amides which may be regarded as completelysubstituted hydrogen substitution products of ammonia were found whollyunsatisfactory for purposes of this invention.

It must be emphasized that it is essential to the invention that theamide content of the recording medium be such that the recording mediumwill remain operative upon prolonged exposure to all ambient atmosphericconditions. Thus the rate of sublimation or evaporation of the amides isa critical factor to be observed in selection of suitable amides for theimpregnant. All the amides found satisfactory for the purposes of thepresent invention have an inherent vapor pressure lower than that ofacetamide. The rate of sublimation or evaporation is of course directlyrelated to the inherent vapor pressure. Thus it may be regarded asessential to the present invention that amides selected as ingredientsof the impregnant according to the present invention have an inherentvapor pressure lower than that of acetamide.

In practicing the invention described herein it is considered within thescope of the invention to fabricate a recording medium containing aplurality of amides having at least four carbon atoms as disclosedherein. It is likewise considered within the scope of the invention toinclude a minor quantity of the order of 15% of one or more amideshaving less than four carbon atoms in the impregnant together with amajor quantity of the order of of one or more amides having at leastfour carbon atoms. Each amide employed in the impregnant must have aninherent vapor pressure lower than that of acetamide and may be either aprimary or secondary amide. Amides suitable as addition ingredients tosupplernent the content of amides having at least four carbon atoms are:urea, propionamide, and derivatives of acetamide such as chloroacetamideand cyanoacetamide.

This application is a continuation-in-part of my application Serial No.102,135, filed June 29, 1949, now abanidoned.

I claim:

1. A dry, white recording medium markable electrolytically only whenheated, comprising a single layer of dry porous material impregnatedessentially with at least one amide having at least four carbon atomsand selected from the group consisting of acetanilide, benzamide,paratoluene sulfonamide, N-ethyl benzene sulfonamide, and butyramide, ahighly ionizable salt, and an electro-responsive substantially colorlessmarking compound.

2. A recording medium in accordance with claim 1 including therewith aminor quantity of an amide having less than four carbon atoms and havingan inherent vapor pressure lower than that of acetamide, the lastmentioned amide being selected from the group consisting of primary andsecondary amides.

3. A recording medium in accordance with claim 1 wherein the markingcompound is a phenol having at least two hydroxy groups in the orthoposition.

4. A recording medium in accordance with claim 1 wherein theelectro-responsive marking compound is sensitive to heat so that when anelectrical current is passed through the medium, the compound changes toa dark color.

References Cited in the file of this patent UNITED STATES PATENTS1,922,463 Turski Aug. 15, 1933 2,316,340 Kohn Apr. 13, 1943 2,419,296Solomon Apr. 22, 1947 2,442,596 Greig June 1, 1948 2,530,956 Gibney Nov.21, 1950 FOREIGN PATENTS 630,146 Great Britain Oct. 6, 1949

1. A DRY, WHITE RECORDING MEDIUM MARKABLE ELECTROLYTICALLY ONLY WHENHEATED, COMPRISING A SINGLE LAYER OF DRY POROUS MATERIAL IMPREGNATEDESSENTIALLY WITH AT LEAST ONE AMIDE HAVING AT LEAST FOUR CARBON ATOMSAND SELECTED FROM THE GROUP CONSISTING OF ACETANILIDE, BENZAMIDE, PARATOLUENE SULFONAMIDE, N-ETHYL BENZENE SULFONAMIDE, AND BUTYRAMIDE, AHIGHLY IONIZABLE SALT, AND AN ELECTRO-RE SPONSIVE SUBSTANTIALLYCOLORLESS MARKING COMPOUND.